1. Field of the Invention
The present invention relates to an adsorption cooling system used for air conditioning apparatus for automobiles, vessels, or residences, or in cooling apparatus of shipping containers for food products or medical products that require refrigeration.
2. Description of the Prior Art
Prior known cooling systems for such air conditioning apparatus and refrigeration apparatus include a well-known heat pump system and, more recently, an adsorption system which uses adsorbents such as zeolite.
The prior technology is exemplified by what is described in Japanese Laid-Open Patent Application 61-139098, filed Jun. 14, 1986, Laid-Open No. 62-5060, laid-open Jan. 12, 1987, priority being claimed on German Patent Application P3521484.8, filed Jun. 14, 1985.
FIG. 15(a) and FIG. 15(b) illustrate a basic principle of a prior known single-adsorber type adsorption cooling system.
In FIG. 15(a), an adsorber 1 is connected with a cooling vessel 2 by a tubular passage 4 having a shutoff valve 3. The adsorber 1 houses adsorbent 1a such as zeolite, and a heat exchange pipe 5 which is in contact with the adsorbent 1a. The cooling vessel 2 contains water as an adsorption medium (adsorbate) which is in thermal contact with a cooling pipe 6. The air which is to be cooled is circulated through the cooling pipe 6. The adsorber 1, the cooling vessel 2 and the tubular passage 4 constitute a closed system which is evacuated of air. A condenser 7 for exchanging heat with the ambient air is provided on the tubular passage 4 near the cooling vessel 2.
With this cooling system, when the valve 3 is opened, the water inside the cooling vessel 2 evaporates into water vapor and flows through the tubular passage 4, in the direction shown by the arrow, into the adsorber 1 to be adsorbed by the adsorbent la by its adsorption action. Because of this, when 14 the water in the cooling vessel 2 evaporates, the latent heat is adsorbed in the cooling vessel 2, so that the temperature in the cooling vessel 2 declines so as to cool the air inside the cooling pipe 6. This operation is called an adsorption process.
Next is an operation whereby the water adsorbed by the adsorbent la is returned to the cooling vessel 2.
Referring now to FIG. 15(b), a high temperature gas from an external heat source is provided through the heat exchange pipe 5 to heat the adsorbent la and thereby desorb and separate the water adsorbed in it. Then, the separated water in a state of vapor is driven through the tubular passage 4, in the direction shown by the arrow, to the condenser 7, where the water vapor becomes liquid water and is recovered in the cooling vessel 2. This operation is called a desorption process.
The adsorption here means a state where the water molecules are retained among the molecules of the adsorbent. Reversely, in the desorption action, the water molecules are desorbed and separated from the molecules of the adsorbent as the adsorbent is heated.
However, the single-adsorber type adsorption cooling system described above is incapable of continuous cooling because the adsorption process and the desorption process must be done alternately in the same system. With this in mind, a dual-adsorber type adsorption cooling system as shown in FIG. 16 has been proposed.
In FIG. 16, two adsorbers 8, 9 are individually connected to a single cooling vessel 14 by tubes 12, 13, that respectively nave shutoff valves 10, 11. The water in the cooling vessel 14 is thermally contacted by a cooling pipe 15 in the same manner as in the case of the single-adsorber type adsorption cooling system described above. Adsorbents 8a, 9a in the adsorbers 8, 9 are also in thermal contact with heat exchange pipes 16, 17, respectively, and condensers 18, 19 are provided on the tubes 12, 13, respectively.
With this cooling system, while the adsorption process is performed at one adsorber, at, for example, the adsorber 8, the desorption process is simultaneously performed at the other adsorber 9. Then, the two adsorbers 8, 9 carry out a switching operation whereby they work in reverse when their respective processes have been completed. Since the adsorber 9 is at high temperature when the desorption operation has been completed, low or ambient temperature air is provided through the heat exchange pipe 17 to cool the adsorbent 9a. A continuous cooling in the cooling vessel 14 is thus made possible by periodically repeating such operation.
(Problems to be Resolved)
In the conventional dual-adsorber type adsorption cooling system described above, when the desorption process is completed in, for example, the adsorber 9, the tube 12 is at a low temperature and the tube 13 is at a high temperature. On the other hand, when the desorption process is completed in the adsorber 8, the tube 12 is at a high temperature and the tube 13 is at a low temperature. Because of this, there is a heat loss in the thermal capacities of the tubes 12, 13 each time when the adsorption-desorption processes are switched over. This heat loss leads to a problem of lowering the coefficient of performance (COP) of the system. Further, the adsorption heat (sensible heat) that is generated in the adsorber performing an adsorption process (adsorbing adsorber) becomes a waste heat. On the other hand, and at the same time, the adsorber performing a desorption process (desorbing adsorber) requires a large amount of thermal energy. This adverse heat requirement situation of the two adsorbers also causes to decrease the system COP.
In the conventional system, successive adsorption-desorption processes are alternated at a constant cycle period. But when automobile engine exhaust gas is utilized as a heating means, the amount of the heat actually varies with time. Therefore, unless the process switchover period is adjusted according to the amount of the available heat, an efficient operation of the system will not be expected and a desirable system COP will not be achieved.
In addition, zeolite, a widely used adsorbent material, will be decomposed and degenerated at a temperature exceeding 500.degree. C., and the water adsorbed in zeolite will not be desorbed and separated at a temperature below 200.degree. C. Therefore, an excessive variation of the available heat is an impediment in the operation of the system. Insufficient amount of available heat will necessitate an extended desorption time, thereby increasing the process cycle period and lowering the operation efficiency of the system.